The best way to do this IMO, and a way that will make you understand what it feels like to be very very confident is to go to a crowded area (mall or whatever) and start singing LOUDLY and dancing like a lunatic for 5-10 minutes.
Do it with a friend or you won't be able to force yourself. The second you pass that "indifference threshhold" you go from zero to hero instantly.
If you can pull this off you'l feel AMAZING probably for a week. You will be untouchable and wil dominate any social situation without even thinking about it. I know because I've done it several times. And I don't care how ****ed up you think you are. Arguably the more insecure and afraid of people's opinions you are the more dramatically you will be confident after you do this exercise.
It basically has the effect of weed x1000 with a ton of energy instead of lethargy thrown in.
Check out my thread for something that will make you confident with a bit more consistency (though not in as dramatic a fashion, more of a buildup type thing)

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